UC San Diego
Selective Area Processing for Semiconductor Device Fabrication
- Author(s): Ahles, Christopher Frederick
- Advisor(s): Kummel, Andrew C
- et al.
As photolithography is reaching its fundamental scaling limitations and transistors are transitioning from 2D to 3D architectures, selective area processing using selective chemistry is becoming a requirement for semiconductor device fabrication. For example, the gate-all-around metal oxide semiconductor field effect transistor (GAA MOSFET) is desirable because it gives better electrostatic control over the channel compared to a conventional 2D MOSFET. However the fabrication of a GAA MOSFET requires the fabrication of a nanowire of the channel material, and this processing step requires selective isotropic etching which can only be achieved via selective chemistry. Chapter 2 of this dissertation is focused on the selective isotropic etching of silicon in preference to germanium and SiGe, which can be used for the fabrication of Ge or SiGe nanowires for Ge- or SiGe- channel GAA MOSFETs.
Chapter 3 of this dissertation describes a selective dry clean which removes native SiOx from Si but does not etch thermal or flowable SiO2. This process is useful, for example, as a preparation step for a selective ALD process which deposits on Si but not on SiO2. A conventional aqueous HF clean would etch the SiO2 as well as the native SiOx and would have to be performed ex situ which would allow for re-contamination of the surface prior to insertion into vacuum. It is shown that careful optimization of a NH3/NF3 based plasma allows for selective removal of native SiOx and gives a cleaner Si/ALD MoSix interface than when aqueous HF is used.
As photolithography reaches its fundamental scaling limit, alternative patterning methods are being investigated. One such method, known as spacer-defined double patterning (SDDP) uses selective ALD to deposit a hard mask with atomically precise thickness. This hard mask can then be used to pattern features much smaller than can be achieved via photolithography. Chapter 4 describes a selective water-free ALD of TiO2 on Si and SiO2 in preference to SiCOH. It is envisioned that this ALD TiO2 can be used as a hard mask in a SDDP fabrication scheme. The fact that it is water-free is important because water can corrode metals and SiCOH at high temperature.